This invention relates generally to the field of geolocation and more particularly to a method and system for determining carrier frequency offsets for positioning signals.
In the application of global positioning system (GPS) technology to the geolocation of wireless devices, a typical geolocation function utilizes a course acquisition (C/A) code, or Gold code, which is received repeatedly from GPS satellites, in order to determine position. In addition to the repeated Gold code sequence, the Gold code comprises satellite message data that is modulated on top of the Gold code signal by inverting the phase according to the message data.
The spectral density level of a signal from a GPS satellite received at a conventional GPS receiver with a direct line-of-sight to the satellite is significantly less than the thermal noise level of the conventional GPS receiver. When satellite signals are received at wireless devices being operated with obstructed views of the sky and thus obstructed line-of-sight, the satellite signals are weakened even further. Consequently, the obstructed signal levels from satellites are generally well below the threshold at which receivers may receive reliable message data signals from the satellites.
Recent solutions to the problem of receiving weakened positioning signals provide for partitioning the geolocation processing functions such that some of these functions are performed at the unknown location which is to be determined and other functions are performed at a location with an unobstructed view of the signal source.
For example, one of these methods, using a satellite signal source, provides for measuring all the satellite signal parameters, including the Doppler shift for each satellite signal, at unobstructed receivers located near the unknown location. The unobstructed receivers then send pertinent data to the unknown location to allow pseudorange estimation to be completed at the unknown location.
Disadvantages associated with this example include a relatively expensive requirement of integration of the network of such receivers with the wireless carrier network that provides a link between the unknown location and the unobstructed receivers. In addition, this solution restricts the joint operation of the unknown location and the unobstructed receivers in performing the geolocation processing functions to only those carrier networks that are so integrated.
The present invention provides an improved method and system for determining carrier frequency offsets for positioning signals. This invention substantially eliminates or reduces disadvantages and problems associated with previous systems and methods. In a particular embodiment, the time to estimate a pseudorange from received positioning signals is minimized, while the processing gain is maximized to facilitate rapid detection of positioning signals while minimizing the consumption of energy.
In accordance with one embodiment of the present invention, a method for directly extracting carrier frequency offsets (CFOs) from positioning signals received at a ranging receiver is provided. The positioning signals comprise a plurality of samples. The method includes, for each of a previously determined number of decimated samples, accumulating a specified number of samples into the decimated sample. A determination is made regarding whether to apply rate correction to the decimated samples. A particular number of rate corrections are applied to each decimated sample to generate a set of results when rate correction is to be applied. Each set of results is stored. A determination is made regarding whether a minimum number of significant carriers exists in the sets of stored results. The CFOs are determined based on the significant carriers when the minimum number of significant carriers exists in the sets of stored results.
Technical advantages of one or more embodiments of the present invention include providing an improved method for determining carrier frequency offsets for positioning signals. In particular, the time required to obtain pseudorange estimates from weakened positioning signals is minimized and the processing gain available in any sample segment used to obtain pseudorange information is maximized. In addition, by using a direct extraction method to quickly determine a CFO, resolving the identity of satellites whose signals are being received, and compensating the received signal by the amount of the CFO, the pseudorange estimation may be accomplished without performing a time-consuming search through the Doppler frequencies and satellite codes.
Other technical advantages of one or more embodiments of the present invention include a geolocation processor that may supply, to a receiver, current fragments of message data that were transmitted during the time the receiver was collecting signal samples. This is possible because the geolocation processor and the receiver are able to exchange time-of-day synchronizing information. As a result, the modulation for the message data may be removed from the samples being processed by the receiver. Accordingly, the available processing gain that can be achieved from a signal averaging process is maximized.
Due to the improvement in processing gain, technical advantages of one or more embodiments of the present invention also include an ability either to obtain greater sensitivity in a given amount of signal processing time or to significantly reduce the amount of processing time to reach a particular level of sensitivity. In this regard, a dynamic process is used that acquires only the amount of signal necessary to achieve reliable detection. This minimizes both the processing time and the amount of intermediate-result memory storage required during signal processing.
Yet another technical advantage of one or more embodiments of the present invention includes the removal of a requirement to integrate a geolocation processing system with the wireless carrier network that provides a communication link between the ranging receiver and an assisting function. Thus, the need for, and the costs associated with, a wireless carrier or plain old telephone system network are eliminated. For example, support from a carrier""s network is not required to determine a coarse position estimation. In addition, a support network of nearby reference receivers, or its equivalent, is not required to provide Doppler-shift compensation or Doppler-shift search assistance. An end-to-end frequency-calibration embodiment in the wireless communication link between the receiver and the geolocation processor to allow making use of the Doppler-shift is also not required.
In addition, technical advantages of one or more embodiments of the present invention include minimized memory use, power consumption and network loading, increased sensitivity, decreased amount of time for a location estimate, and greater privacy for customers.
From the following figures, description, and claims, other technical advantages of the present invention will be readily apparent to one skilled in the art.